Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers, in particular, convert electrical signals to acoustic waves and acoustic waves to electrical signals using inverse and direct piezoelectric effects. Acoustic transducers generally include acoustic resonators, such as bulk acoustic wave (BAW) resonators and surface acoustic wave (SAW) and may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, FBARs may be used for electrical filters and voltage transformers. Generally, an acoustic resonator has a layer of piezoelectric material between two conductive plates (electrodes), which may be formed on a thin membrane. FBAR devices, in particular, generate acoustic waves that can propagate in lateral directions when stimulated by an applied time-varying electric field, as well as higher order harmonic mixing products. The laterally propagating modes and the higher order harmonic mixing products may have a deleterious impact on functionality.
FIG. 1 depicts a known FBAR 100. A first electrode 102 is provided over a substrate 101. A piezoelectric layer 103 is provided over the first electrode 102, and a second electrode 104 is provided over the piezoelectric layer 103. A cavity 105 is provided in the substrate 101, allowing confinement in the vertical direction (y-direction in the coordinate system of FIG. 1) of thickness extensional (TE) modes to the membrane structure. A connection side 106 of the FBAR 100 allows for electrical signals to be provided to/from the second electrode 104.
As is known, the active region of the FBAR comprises the area of overlap of the first electrode 102, the piezoelectric layer 103, and the second electrode 104 over the cavity 105. To improve performance of the FBAR 100 (as measured by certain quantities such as the quality factor (Q) of the FBAR 100), it is desirable to reduce loss of acoustic energy from the active region. Notably, it is useful to minimize the overlap of first electrode 102, piezoelectric layer 103 and second electrode 104 that extend beyond the cavity 105 (i.e., over the substrate 101), which are referred to as “dead” or “inactive” FBAR regions as these can result in loss of acoustic energy to the substrate 101. Moreover, it is useful to reduce scattering points caused by acoustic impedance discontinuities.
Vertical lines 107, 108, 109 and 110 depict locations of planes where acoustic impedance discontinuities exist between various layers of the FBAR 100. At each of the acoustic impedance discontinuities, electrically excited propagating and evanescent TE modes undergo reflection of the propagating TE mode back to the active region of the FBAR 100, and scattering of both propagating and evanescent TE modes to unwanted shear and flexural modes. Illustratively, at the impedance discontinuities depicted by vertical lines 107, 108, 109 and 110, reflected acoustic energy is depicted by arrows 112, and scattered acoustic energy is depicted by arrows 113 and 114. Specifically, at the termination of the second electrode 104 at vertical line 107, acoustic energy is reflected (arrow 112) and scattered (arrow 113). Similarly, at the edge of the cavity 105 an acoustic discontinuity (vertical line 108), results in reflection (arrow 112) of acoustic energy back to the active region of the FBAR 100 and scattering (arrow 113).
A transition region 111 at connection side 106 is also depicted in FIG. 1. The transition region 111 comprises a slope in the piezoelectric layer 103 and the second electrode 104 that is created by a slope 115 at the termination of the first electrode 102. The first electrode 102 is terminated at slope 115 to reduce the area of the “dead” FBAR outside the cavity 105. While the first electrode 102 terminates, the piezoelectric layer 103 is grown over the substrate 101, and the second electrode 104 is formed over the piezoelectric layer 103 at connection side 106. The piezoelectric layer 103 is grown over the substrate 101 as shown to reduce the occurrence of defects in the layer that can result from abrupt changes in the contour. In furtherance of this, the first electrode 102 does not abruptly terminated, but rather is terminated by the slope 115. Although the slope 115 fosters a reduction of defects in the piezoelectric layer 103, the overall sloping of the first electrode 102, the piezoelectric layer 103 and the second electrode 104 in the transition region 111 results in impedance discontinuities at each change in contour as represented by vertical lines 109, 110. These impedance discontinuities result in scattering of acoustic energy (arrow 113) out of the active region and the scattering of acoustic energy (arrow 114) into the substrate 101. The “sloped” edges created in the first electrode 102, the piezoelectric layer 103 and the second electrode 104 along the connection side are particularly problematic because of the enhanced scattering of acoustic energy (arrow 114) into the substrate.
What is needed, therefore, is a structure useful in mitigating acoustic losses at the boundaries of the BAW resonator to improve mode confinement in the active region of the FBAR.